Large concrete products are typically cast in molds. The process of casting is used to make large concrete products such as beams for use in highway bridges, tunnel liners, building construction and the like. Many of these concrete products have tensioned steel strands therein to prestress the concrete product. The steel strands are placed in the mold and tensioned before the concrete is poured in the mold. As the concrete cures, the steel strand and concrete bond and the tension in the strand creates the prestress in the concrete product. Each of the strands is typically tensioned by 30,000 pounds force. Often, the strands are also tensioned perpendicular their length into a slight V shape near the middle of the mold to provide negative loading at the top of the concrete product.
In a self stressing mold, bearing plates(sometimes referred to as jacking plates) are placed at the ends of the mold and the ends of the strands in the mold pass through aligned holes in the plates and extend outwardly from the plates a length sufficient to allow a hydraulic cylinder or other tensioning device to grasp an end of the strand to tension the strand. Once tensioned, conical wedge type strand chucks acting between the strand and the plates maintain the tension. Because the tension in the strands is passed through the plates, and the plates engage the mold, the tension, in turn, passes through the mold. In such a design, the mold must be sufficiently strong to absorb these stresses.
In other applications, external abutments may be provided at each end of the mold to tension the strands passing through the mold. The external abutments are supported in the ground at the mold site or supported by other structures. In this design, no bearing plates are necessary. The mold is not exposed to the tension forces in the strands and consequently need not be designed to withstand those stresses.
A typical concrete product is a T or double T molded in a long T or double T-shaped mold which may use 6 tensioning strands in each leg of the T(for a total of 12 in a double T), for example. The mold is often sufficiently long to mold a number of concrete products simultaneously therein along the length of the mold. For example, a mold may be over 400 feet long, and used to mold up to ten 40 foot long T or double T concrete products simultaneously. The ends of the concrete products are formed by headers or bulkheads inserted into the mold at the desired spacing to confine the liquid concrete as it is poured into the mold. The concrete products are commonly reinforced by rebar or mesh. Commonly, such T or double T molds are self stressing and the concrete products are prestressed by tensioned steel strand passing through bearing plates at the ends of the mold, the headers and the concrete product from end to end, which bonds to the concrete as the concrete cures. The bearing plates hold and distribute the tensioning forces in the steel strand. The bearing plates are typically steel about 4 inches thick to resist the tensioning forces exerted.
Molds for a large, 120 foot long highway I beam, using perhaps 60 separate steel strands, each ⅜, ½, or {fraction (9/16)} inch in diameter, for example, are not self stressing. The strands are drawn through the mold(passing through aligned holes in any headers used) and tensioned between external abutments.
As each strand will usually be at least as long as the mold, say 400 to 500 feet, with some extra length to extend out the ends of the bearing plates or to the external abutments, the difficulty of manipulating such strand lengths can be appreciated. In the past, if 60 strands were needed in the concrete product, 60 separate strand packs could be positioned at the mold site. Similarly, a double T may use 12 separate strands, 6 for each of the two legs of the double T, requiring 12 separate strand packs to be positioned at the mold site. The strands are pulled through the mold simultaneously. Once the strands are in place, the strands are cut off the strand packs adjacent the bearing plate or external abutment and tensioned. Each strand pack typically has about 12,000 feet of strand(the actual length dependent on the diameter of the strand). For ten 40 foot T concrete products molded simultaneously in a mold somewhat over 400 feet long, only a little more than 400 feet of strand off of each of the strand packs would be needed for the concrete products. Thus, expensive inventory, represented by the unused strand on the strand packs, is idle as the concrete products cure.